A drum brake comprises a drum and a brake backing plate. The drum and the backing plate are circular in shape. The backing plate is designed to be inserted in the drum.
A drum brake is fixed firstly concentrically to the rim of at least one wheel of a vehicle by means of the drum. The drum brake is also fixed to an axle of the vehicle via the backing plate.
The drum and the backing plate come into contact with each other via two brake shoes. The two brake shoes are in the form of circular arcs and they are disposed diametrically opposite each other. Together they define a circle of a certain diameter. On the outside faces of the brake shoes, and facing the overlying periphery of the drum, there is disposed a brake lining. The brake lining is designed to be pressed against the overlying periphery of the drum in order to brake the wheel of the vehicle. Each of the two shoes has hinged thereto a hand-brake lever and a lever for taking up slack. The hand-brake lever is pulled by a hand-brake cable, and the cable is actuated under the control of a driver. The hand-brake lever is designed to exert force on each of the shoes in such a manner as to cause the shoes to move apart from each other and press against the overlying periphery of the drum. The slack-takeup lever is designed to move the two brake shoes towards the periphery of the drum as the brake linings become worn.
In some cases, a brake actuator can be installed at one end of the shoe, at the location where two brake shoes meet. The brake actuator is also designed to exert force on both shoes, in response to the driver depressing a brake pedal, so that the shoes move apart from each other and come to press against the periphery of the drum.
A brake drum is assembled by inserting the backing plate carrying the two brake shoes, the hand-brake lever, the slack-takeup lever, and the braking actuator into the inside of the drum. Such assembly requires assembly clearance for the two brake shoes. The assembly clearance of the two brake shoes corresponds to a distance between a given point on the periphery of the drum and a corresponding other point on the circle defined by the two brake shoes, with the average distance needed to enable the two brake shoes to be inserted inside the drum being about 0.8 millimeters (mm). Once inside the drum, the two brake shoes need to be moved apart so as to take up operating clearance. Operating clearance corresponds to the optimum spacing for the brake shoes relative to the diameter of the drum. This optimum spacing is such that at rest there is no contact between the brake shoes and the drum. Nevertheless, this optimum clearance must be close enough to the overlying periphery of the drum so that for a given level of braking, braking takes place relatively immediately. By way of example, the operating clearance or the distance between the outside diameter of the brake shoes and the diameter of the drum is about 0.3 mm to 0.4 mm.
To go from assembly clearance to operating clearance, it is known to make use of the brake pedal and the brake actuator. For this purpose, an operator applies several consecutive presses to the brake pedal, thereby causing hydraulic action to be taken. While this hydraulic pressure is being applied, the brake shoes move apart from each other, and the slack-takeup device engages a non-return ratchet which little by little prevents the shoes from returning to a rest position that is too far away. Several applications of hydraulic pressure are needed and they are performed at the end of the assembly line. In one example, the operator needs to apply hydraulic pressure a score of times using the brake actuator in order to reduce the assembly clearance from about 0.8 mm to an operating clearance of 0.3 mm to 0.4 mm. Applying hydraulic pressure in this way is tedious.
In addition, over time with the drum brake being used, the brake linings wear. The brake linings become worn because they are pressed against and rub against the periphery of the drum during braking. The distance between a point on the circle defined by the brake shoes and a corresponding point on the periphery of the drum increases. This distance is subsequently regularly re-adjusted, to within operating clearance, by the slack-takeup lever.
However, the friction between the brake linings and the periphery of the drum can over time, and because of use, lead to the wall of the drum being dug away. This forms a step in the wall of the drum. Because such a step is formed and because of the increase in the spacing between the two brake shoes due to automatic and regular readjustment of the distance between the circle defined by the brake shoes and the periphery of the drum, it can become more difficult, or even impossible to extract the two brake shoes from the drum.